The Optical Signal-to-Noise Ratio (OSNR) is a direct measure of the quality of signal carried by an optical telecommunication link. Under normal and proper operating conditions, the OSNR of an optical communication link is typically high, often in excess of 15 dB or 20 dB, or even greater. The dominant component of the noise in an optical communication link is typically unpolarized Amplified Spontaneous Emission (ASE), which is a broadband noise source contributed by the optical amplifiers in the link. In general, the ASE may be considered to be spectrally flat across the small wavelength range spanning the full signal spectral width, provided that there is no spectral filtering in the vicinity of the signal.
The IEC 61280-2-9 Fiber-optic communication subsystem test procedures—Part 2-9 standards (ed. 1.0 b:2002) provides a standard method for determining OSNR in Dense Wavelength Division Multiplexing (DWDM) networks. This method is based on the assumption that the interchannel noise level is representative of the noise level at the signal peak position. The method interpolates the power level of the noise outside the signal bandwidth to evaluate the in-band noise in the signal bandwidth. Increased modulation rates, which enlarge the signal bandwidth, and increased channel density, reduce the interchannel width; therefore resulting in severe spectral characteristics requirements for the optical spectrum analyzers used to perform the measurement. The procedures described in the standards are able to cope with these difficulties when the noise level of adjacent peaks is mostly continuous. For example, the standards propose a two-scan procedure to first measure a broad modulated peak with a larger resolution bandwidth to capture the entire signal peak and then determine the noise using a narrow resolution bandwidth to minimize the contributions of the main and adjacent peaks on the interchannel noise level. Alternatively, commercial Optical Spectrum Analyzers (OSA) (such as EXFO's FTB-5240, in its versions available before 2007) implement a related procedure by performing an integrated peak calculation and fine noise determination in a single scan.
However, to strictly comply with the standards recommendation, the noise level should be determined at the mid-channel spacing between peaks. In the case where noise is spectrally filtered with the signal peak, for instance, after passing through multiplexers or demultiplexers—such as Reconfigurable Optical Add Drop Multiplexers (ROADM)—the mid-spacing noise level is no longer representative of the in-band noise level, which is the relevant parameter for the OSNR determination. The interpolation of the interchannel noise level then becomes unreliable. This can be mitigated by relying on a very sharp spectral response of the OSA filter and adaptive processing to determine the noise level at the shoulders where the noise meets the base of a signal profile within the channel bandwidth. However, increased modulation rates combined with narrow filtering of multiplexers and demultiplexers is making it increasingly difficult to achieve a reliable measurement of the noise level within the channel bandwidth.
Active polarization-nulling (see J. H. Lee et al, “OSNR Monitoring Technique Using Polarization-Nulling Method”, IEEE Photonics Technology Letters, Vol. 13, No. 1, January 2001) provides an alternative to a direct analysis of the optical spectrum. This method uses the fact that the signal peak is generally polarized while the noise is generally unpolarized. Using a polarization controller cascaded with a polarizer (the latter serving as an analyzer), it is possible to actively control the polarization of the input signal in order to find a condition where the signal peak is maximally suppressed by the polarizer. An optical spectrum trace is acquired while the signal peak is suppressed and reveals the in-band noise within the optical channel bandwidth. The noise level within the optical channel bandwidth can be determined using the acquired optical spectrum trace.
Variants to the active polarization-nulling method are described in U.S. Pat. No. 7,106,443 to Wein et al.; in Sköld et al., “PMD-insensitive DOP-based OSNR monitoring by spectral SOP measurements”, Paper OThH3, Optical Fiber Communications Conference, Anaheim, USA, March 2005); and in U.S. Pat. No. 7,756,369 to Rudolph et al.
The active polarization-nulling method and its variants all require that the polarized signal peak be suppressed at or very close to zero. In practice, this requires a degree of extinction of the signal peak which is at least 10 dB greater than the highest possible OSNR to be measured. For example, for measuring an OSNR of 25 dB within an accuracy of 0.5 dB, a 38-dB extinction is required. This high degree of extinction imposes constraints on the instrumental noise floor that normally is often limited by the electronics, quality of the polarization-diversity optics, etc., which, in order to be satisfactorily overcome, requires increasing the inherent cost of the instrument. Notwithstanding the aforementioned instrumental constraints, attainment of such a high extinction ratio also requires either an excellent coverage of the States-Of-Polarization (SOPs) on the Poincaré sphere, i.e. the generation of a very large number of SOPs or the use of a full “high-end”, i.e. very accurately calibrated, and hence costly polarimetric OSA.
It is noted however that the limiting noise source in most optically-filtered long-haul optical networks is the signal-ASE beat noise, in which the signal and the ASE interfere at baseband frequencies within the electronic detection bandwidth. In typical optical communications systems employing optical amplifiers, signal-ASE beat noise is the limiting noise term for optical performance, and can be directly related to the Bit Error Rate (BER) of the optical communication channel. Thus, estimation of the in-band OSNR provides an indicative measure of the system performance. However, new systems are currently being developed and deployed which exploit multi-bit-per-symbol advanced modulation formats to transmit more than 100 Gbit/s, with symbol rates of 27 GBaud and higher. Not only are the associated optical spectra of the modulated signals much wider than previous (generally on-off-keying) 10 Gb/s systems, but the spectral profiles are often more complicated, and not necessarily “sharply peaked” at the center. Hence, accurate signal-ASE beat noise estimations may require a convolution of the superposed (or “underlying”, a less rigorous but widely employed terminology) optical noise spectral trace with the signal spectral trace. In tightly filtered systems, this underlying noise is itself often filtered over a significant portion of the channel bandwidth, near the filter edges. Accordingly, OSNR of such systems can not be determined reliably based on an estimation of the underlying in-band noise assuming a flat optical noise spectral trace.
There is thus a need for reliably determining the optical noise spectral trace underlying the optical signal peak. In particular, there is a need of methodology that is applicable in the case of DWDM networks, where individual channels may carry respective signals that have traversed different optical links and hence have different underlying noise properties.